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LMK00334-Q1: Clock & timing forum

Steve Chang70
Prodigy 170 points
Part Number: LMK00334-Q1
Other Parts Discussed in Thread: LMK00334

Dear Experts,

We have a design with LMK00334 to distribute PCI Express clock for the PCI Express devices. We can measure the 100MHz differential at one of its input. But there is no clock on any of its output. Attached is the schematics for you. Please note that all AC coupling capacitors were installed with 0 Ohm resistors.

Steve

  • 0
    TI__Genius 16845 points

    Hi Steve,

    I would expect you to measure clock outputs if you can measure the input. One thing to confirm is that the inputs and outputs are properly terminated. Standard HCSL drivers expect a 50ohm termination to GND (see datasheet figure 19). If the driver does not have integrated termination, you must have the termination resistors placed externally. 

    Kind regards,
    Lane

  • 0 in reply to Lane Boyd
    Prodigy 170 points

    Hi Lane,

    In our application, the LM00334 receives the 100MHz differential REFCLK+/- from the standard PCI Express slot, in that case, do we need to add 50 ohm resistors to GND too?

    Another question is the outputs of the LMK00334, in our application, they are driving the standard PCI Express slots directly, in that case, do we need to put the 0.1uF AC coupling capacitors in between?

    I also have a question why all the PCI Express clock buffers other than TI's which we have been used do not need add extra 50ohm resistors to GND, Why does LMK00334 need?

    Perhaps I don't have precise knowledge about the standard PCI Express REFCLK+/- (is it HCSL? or LVPECL、LVDS、CML? or others?), please explain above questions as detail as possible. Thank you!

    Steve

  • 0 in reply to Steve Chang70
    TI__Genius 16845 points

    Hi Steve,

    When you measure clock input at the LMK00334, are you using an oscilloscope? Is the scope channel configured as 50ohm or 1Mohm?

    Standard HCSL drivers require a 50ohm termination to GND. It must be a DC path to GND as 14mA drive current sets the swing and common mode level assuming 50ohm termination.

    If you can measure a good clock input 50ohm channel but not 1Mohm, then the 50ohms are required. 

    The AC-coupling capacitors are not needed, but there should be no functional difference regardless of whether it is 0ohm or AC-coupling cap.

    We do offer some clock devices with integrated output termination, LMK00334 does not have integrated output termination. Integrated terminations have become more popular due to simplicity. In the past typical HCSL drivers expected the termination component at the receiver for best signal integrity.

    PCIE Express REFCLK is is typically 100MHz HCSL.

    I hope this information helps

    Kind regards.
    Lane

  • 0 in reply to Lane Boyd
    Prodigy 170 points

    Hi Lane,

    I understood. Thanks for your explanation. Have a great day!

    Steve